VERFAHREN ZUR HERSTELLUNG VON UNGESAETTIGTEN CARBONSAEUREN

The gas phase catalytic oxidation of an unsaturated aldehyde with molecular oxygen at 200 DEG to 500 DEG C. to give the corresponding unsaturated carboxylic acid is conducted in the presence of a catalyst represented by the following formula: Mo12PaXbYcZdOe wherein the subscripts represent the atomic ratios of each component and a is 0.5 to 6, b is 0.001 to 6, c is 0.2 to 6, d is 0 to 6, and e is a value determined by the valencies of the elements present in the catalyst; and Mo is molybdenum, P is phosphorus, O is oxygen, X is at least one metal selected from the group consisting of rhodium, cerium and zirconium, Y is at least one alkali metal selected from the group consisting of potassium, rubidium and cesium, and Z is at least one metal selected from the group consisting of iron, cobalt, nickel, zinc, antimony, silicon, bismuth, cadmium, uranium, manganese, copper, vanadium, niobium and tantalum. This catalyst is especially effective for the preparation of methacrylic acid from methacrolein and has a very long life time.